SYNOPSIS OF GENERA. 
XXV 
Microdon, Conrad (Journal of the Academy of Natural Sciences, Philadelphia, 
vol. viii, p. 247. 1842). [Type, Microdon bellistriata, Conrad.] 
Shell equivalve, inequilateral, transversely sub-elliptical or sub-quadrate in 
outline. Cardinal line straight or slightly arcuate. Anterior end narrowed 
and rounded. Posterior extremity broad and truncate. Beaks small, pointed, 
appressed, situated nearer the anterior end. Umbonal ridge more or less 
defined, extending from the beaks to the post-inferior angle. 
Surface marked by more or less rugose and regular concentric strise. 
Hinge characterized by a triangular tooth in each valve, that of the left 
valve short and situated beneath the beak, with a more elongate pit or groove 
behind it for the reception of the tooth of the right valve. The right valve 
has also a triangular pit beneath the beak for the reception of the short tooth 
in the left valve, and a longer triangular fold behind, which is sometimes 
double. No lateral teeth have been observed, unless the long oblique fold of the 
right valve be regarded as a lateral tooth. Ligament external, extending almost 
or quite the entire length of the cardinal line. Anterior muscular impression 
moderately large, sub-circular or short reniform; posterior scar superficial. 
Pallial line simple. 
Owing to the fact that the name Microdon had previously been established by 
Agassiz for a genus of fishes, the name Eodon was proposed by Hall and Micro- 
donella. by (Ehlert. The genus Cypricardella (Trans. Albany Inst., vol. iv. 
1856) has been shown to be identical with Microdon * If the name Microdon 
is to be suppressed, the name Cypricar della will have precedence. Microdon is 
retained for the present, using the genus Cypricardella parenthetically. 
Examples : Microdon ( Cypricardella ) bellistriatus, pi. lxxiii, figs. 7-22 ; pi. 
lxxiv, figs. 5-10. 
Microdon ( Cypricardella) tenuistriatus, pi. lxxiii, figs. 23-30; pi. 
lxxiv, figs. 20, 21. 
* R. P. Whitfield. Bull. Am. Mus. Nat. Hist., vol. i, No. 3, p. 03. 1882. 
D 
